In general, a function of preventing deterioration in the quality of contents is required of packaging materials. Particularly, in a field of food packaging materials and the like, in which contents are susceptible to deterioration by oxidation, the packaging materials are required to have excellent oxygen gas barrier property.
At present, for example, PVA films, films formed of a partially saponified product (EVOH) of an ethylene-vinyl acetate copolymer, polyvinylidene chloride (PVDC) films and the like are commonly used as films excellent in oxygen gas barrier property in the field of the packaging materials. Of these, the PVDC films have a feature that their gas barrier property scarcely depends on humidity unlike the PVA films and EVOH films. However, when they are incinerated, chlorine gas is generated, and so they involve an environmental problem.
The PVA films have a feature that they have good flexibility, antistatic property and oxygen gas barrier property, and moreover are best in oxygen gas barrier property among general synthetic resin films in a dry state. However, they involve a drawback that their oxygen gas barrier property is impaired to a great extent due to moisture absorption under high-humidity conditions, and moreover they are easy to dissolve in boiling water. Therefore, films enhanced in crystallinity by a heat treatment or improved in crystallinity and mechanical strength by biaxial stretching in a final stage of film processing are marketed as general-purpose PVA films. Besides, in order to lessen the dependence of the gas barrier property on humidity, some PVA films are provided in the form of a laminate film having a multi-layer structure. The PVA films according to the conventional processes are however still insufficient in the prevention of deterioration in gas barrier property due to moisture absorption and improving effect as to water resistance.
In U.S. Pat. No. 2,169,250, it has been disclosed to form films, fibers and the like from an aqueous mixture solution containing PVA and a polycarboxylic acid, and subject the formed products to a heat treatment so as to react PVA with the polycarboxylic acid, thereby introducing a cross-linked structure into the formed products to make them insoluble in water and most solvents. As a specific example thereof, it has been described to polymerize a methacrylic acid monomer in an aqueous solution of PVA, cast the resulting reaction mixture on a glass plate, dry the resulting film, and then heat the dry film for 5 minutes at 140.degree. C., thereby obtaining a transparent crosslinked film (Example I). However, any film exhibiting excellent oxygen gas barrier property under high-humidity conditions cannot be obtained by the heat-treating conditions disclosed in this reference.
On the other hand, there have been proposed films and sheets making use of a mixture of PVA and polyacrylic acid (for example, Japanese Patent Publication No. 47743/1988, and Japanese Patent Publication Nos. 14376/1990 and 27941/1990). All these films and sheets are water-soluble or water-absorbable and not films having good water resistance and oxygen gas barrier property.
Poly(meth)acrylic acid (i.e., polyacrylic acid or polymethacrylic acid) or a partially neutralized salt thereof is a water-soluble polymer and may be formed into a film from its solution by a casting process. The resultant film is good in oxygen gas barrier property under dry conditions. However, this film is unsuitable for use as a packaging material for food or the like containing a great amount of water because it shows strong hydrophilic nature.
The present inventors repeatedly carried out an investigation with a view toward producing a film which can exhibit excellent oxygen gas barrier property inherent in a PVA film even under high-humidity conditions and moreover has excellent water resistance. As a result, it was found that when a film is formed from a mixture containing PVA and poly(meth)acrylic acid or a partially neutralized salt thereof in a specific proportion, and the resultant film is subjected to a heat treatment under specific conditions, these objects can be achieved (U.S. patent application Ser. Nos. 08/185,353 and 08/310,749). However, the simple film thus obtained is insufficient in sealing property, mechanical strength, moisture resistance, etc., which are required of packaging materials. There has thus been a demand for a further improvement.